Branched distribution circuits, for purposes of this disclosure include any distribution circuits containing Ys, Ts or other branches. Branched circuits specifically include the normal neighborhood underground distribution system and, another common example which is utilized in this disclosure for purposes of explanation, is a network distribution circuit. Network electrical distribution circuits is a term utilized to describe power distribution systems in densely populated areas, such as inner cities, large buildings, etc. A single network electrical distribution circuit is typically three to five miles in total length, includes several branches and 50 or more transformers. Network electrical distribution circuits are designed to provide high service reliability. The distribution circuits accomplish this by routing two or more high (primary) voltage feeder cables from a single substation to a common usage point. At the common usage point, network transformers step down the primary voltage to levels commonly used in buildings, commercial establishments, and the like. The lower (secondary) voltages, which is typically 480 volts, are tied together on a common bus bar at the point of usage. In this manner, if one network transformer fails, then the remaining transformers can carry the load without any interruption to the customer.
A further feature of network electrical distribution circuits is that the network transformers are protected by devices called secondary network protectors. These secondary network protectors sense any secondary current back-flow from the secondary bus bar towards the primary side of the network transformer windings. This current back-flow is exactly the condition that occurs when a fault appears on a feeder cable. Thus, when a secondary network protector senses a current backflow condition, the current flow through that feeder is abruptly halted. When a fault (e.g., a path to ground) occurs on one feeder, other feeders connected at the same secondary bus bar attempt to back-feed current through the network transformer connected to the faulted feeder and, hence, to ground through the fault location. The secondary network protectors prevent this current back-flow by automatically sensing the back-flow condition and opening switches on the secondary side of the network transformers connected to the faulted feeder.
When a fault occurs on a network electrical distribution circuit, it compromises the redundancy afforded by the basic network design by taking away one of the feed sources to the common usage point. Accordingly, utilities must repair the fault as rapidly as possible in order to regain the design redundancy. Before repairs can begin, however, the exact location of the fault must be determined.
Currently, utility personnel locate faults in network electrical distribution circuits by using a device known in the industry as a "thumper". A thumper is basically a large capacitive discharge device that stores dc voltage (current) and periodically (e.g., every three seconds) discharges the stored energy into the distribution circuit. Energy introduced during the thumping process causes an audible "thump" at the fault location. The fault location is then determined by personnel positioned to hear and pinpoint the thump origin.
Thumping cables generally causes further damage to cables that are already in a weakened state, as evidenced by the existence of the fault. Thumping is also a time consuming operation which is counter to the goal of repairing the fault as quickly as possible.